The field of this invention relates to synthetic linear polyamide polymers and copolymers characterized by an improved impact resistance and which are thermoplastic and easily moldable by injection or vacuum molding techniques or by extrusion to form a large variety of useful shaped articles.
Linear fiber-forming polyamides suitable for use in manufacture of useful textile fibers have been made by condensing terephthalic acid with polymethylenediamines. These polyamides are typical nylon polymers, being opaque, highly crystalline, easily drawn to form useful textile fibers and melting over a narrow temperature range.
The drawback of such resins, which are known for their properties such as hardness, high tensile strength, toughness and rigidity, is their generally high rigidity resulting in low impact strength, especially at low temperatures.
In the past, it has been proposed to combine linear, crystalline polyamides with other hydrocarbon polymers to obtain less rigid and more flexible polyamides. While an improvement was accomplished in these properties of these highly crystalline nylon polymers, a deterioration of other properties was observed compared with the properties of original crystalline polymer. In one proposed method, U.S. Pat. No. 3,150,117 describes amorphous copolyamides based on aromatic dicarboxylic acids with aryl-substituted hexamethylenediamine. The copolyamides are made by condensing terephthalic acid, isophthalic acid and mixtures thereof with a hexamethylenediamine containing at least three side chain carbon atoms introduced by alkyl substitution of the main hydrocarbon chain. Suitable hexamethylenediamines taught are 2-methyl-4-ethylhexamethylenediamine; 2-ethyl-4-methylhexamethylenediamine; 2,2,4-trimethylhexamethylenediamine; 3-isopropylhexamethylenediamine; etc. The polyamides produced are amorphous, are soluble in typical polyamide solvents such as sulfuric acid, formic acid, phenol and cresol, and are transparent and thermoplastic but have a low capacity for elongation. The polyamides have similar properties whether made with terephthalic acid, isophthalic acid or with mixtures of the two. However, the polyamides with no more than 10% of isophthalic acid are preferred because of noticeable deterioration in mechanical properties such as tensile strength, impact resistance, flexural strength, etc. with acid mixtures containing more than 10% isophthalic acid. Melting points of polyamides made from mixtures of acids of compositions over 20% isophthalic acid dropped sharply and did not go through an eutectic point. The copolyamides are suitable for forming shaped articles and, when liquified in solvents, may be useful in coatings or adhesives, but do not form useful textile fibers because of their amorphous state.
In another proposed method, U.S. Pat. No. 4,210,742 describes transparent polyamides having lower water absorption, increased resistance to hydrolysis, good dimensional stability under the influence of moisture and correspondingly improved mechanical and electrical properties. These polyamides are obtained by reacting isophthalic acid or derivative thereof and, if desired, terephthalic acid and/or derivative thereof and a diamine substituted with alkyl or cycloalkyl groups. U.S. Pat. No. 4,210,743 teaches a similar process to obtain transparent copolyamides with improved mechanical properties obtained by reacting salts of isophthalic acid and/or salts of terephthalic acid with substituted diamines.
High impact predominantly crystalline nylon molding compounds with higher Izod impact resistance are obtained in molded objects by compositions of crystalline polyamides with other modifying groups in the molecule. U.S. Pat. No. 3,388,186 teaches that compositions of crystalline nylon-6 containing ethylene copolymer having a minor proportion of acrylic or methacrylic acid or alkyl ester thereof as backbone and having grafted polycaproamide side chains show higher impact resistance than nylon-6 molding compositions not containing the above graft copolymer. U.S. Pat. No. 3,472,916 teaches blends of 70-98% by weight crystalline nylon-6 homopolymer with 2-30% by weight of ethylene/acrylic or methacrylic alkyl ester copolymer having improved Izod impact resistance, compared to nylon-6 alone. U.S. Pat. No. 3,963,799 teaches ternary blends of crystalline polyamide, polyethylene or copolymer thereof and an amount of copolymer having ethylene backbone and polycaproamide side chains. The graft copolymers are considered to be anchored within the polymer molecule by amine reactive sites provided by the comonomer.
Other blends of polyamides and polyolefins are known. U.S. Pat. No. 3,093,255 teaches a process for preparing a blend wherein each component is present in the amount of at least 5 percent by weight. The polyamides are designated as being nylon-6 or nylon 66. Blends of modified polyethylene and a polyamide are known to make hot melt coatings and transparent packaging materials as taught in U.S. Pat. No. 3,484,403. U.S. Pat. No. 3,626,026 describes a blend of an ethylene copolymer and a polyamide used as hot melt compositions. The ethylene copolymer contains vinyl acetate or ethyl acrylate. Other blends have also been disclosed such as a polyamide-ethylene copolymer blend containing diphenyl carbonate (Netherlands Pat. No. 71-03319 to Imperial Chemicals Inds. Ltd.); polyolefin and polyamide blends with sulphonate as crystallization accelerator (German Pat. No. 2,002,650); a homogeneous polyamide-polyolefin blend containing an oxidized wax to improve impact strength (German Pat. No. 2,038,317); a polyamide and polyolefin blend, a copolymer obtained by grafting an unsaturated dicarboxylic acid into an olefin polymer, and a compound which can be an aromatic carboxylic acid. (British Pat. No. 1,403,797).
Accordingly, a number of methods and compositions have been taught to obtain polyamides of modified properties, including impact resistance. Specific teachings have been directed to modifying amorphous polyamides by introducing modifying groups into the polymer molecule. Crystalline polyamides such as nylon 6 and nylon 66 are taught as being modified to obtain desired mechanical properties through use of blends with other polymers with modified physical characteristics. However, compositions of amorphous polyamides which are thermoplastic and characterized by an improved impact resistance and which comprise terephthalamide and/or isophthalamide polymers physically blended with polyolefin polymers have not been previously known.